In general, an insulated electrical conductor wire, the conductor of which is coated with heat resistant organic resin such as polyimide, fluororesin or the like, has been employed in equipment such as heating equipment or a fire alarm, for which a safe operation at a high temperature is required. Such wires are also used in the environment of an automobile, particularly in its engine compartment, which is heated to a high temperature, particularly when these wires come into contact with the engine.
Further, an insulated wire, the conductor core of which is passed through a ceramic insulator tube, or an MI cable (Mineral Insulated Cable) having a conductor passing through a heat resistant alloy tube of a stainless steel alloy filled with metal oxide particulates of magnesium oxide or the like, has been employed in a case for which a particularly high heat resistance is required or in an environment for which a high degree of vacuum is required.
On the other hand, a fiberglass braided insulated wire employing a fabric of glass fibers as an insulating member or the like, can be mentioned as an insulated wire having a desirable flexibility, and which can be used in a high-temperature environment. As an insulated wire which has an excellent heat resistance, electrical insulation and ability to dissipate heat there exists the so-called alumire wire, which is produced by anodizing a wire of an aluminum alloy.
It has also been proposed to produce an insulated wire by employing a material such as a metal alkoxide or a metal organic acid salt, that is changeable into a ceramic state by heating for forming a ceramic film around a conductor.
In the aforementioned insulated wire with a conductor core coated with a heat resistant organic resin, the temperature under which the insulation can be maintained, is about 300.degree. C. at the most. Therefore, it has been impossible to use such an insulated wire where a good insulation is required even at a higher temperature.
On the other hand, the aforementioned insulated wire with a conductor passing through a ceramic insulator tube, has a disavantage in that it has an inferior flexibility although its insulation can be maintained at a high temperature. Further, although the aforementioned MI cable can maintain its insulation at a high temperature and is flexible as compared with the aforementioned wire with a conductor passing through a ceramic insulator tube, it is difficult to bend such an MI cable even with large curvature, not to mention a small bending curve.
Further, the aforementioned fiber-glass braided insulated wire can maintain its insulation even at a high temperature and it has an excellent flexibility. However, it has been impossible to use this wire in an environment for which a high degree of vacuum must be maintained, since the fiberglass insulation easily discharges dust.
On the other hand, the aforementioned alumite wire can maintain its insulation even at a high temperature, and has some flexibility. However, the use of the alumite wire has been limited since the conductor employed in an alumite wire is restricted to aluminum alone.
As to the aforementioned insulated wire which is made by forming a ceramic layer around a conductor, the ceramic layer is mostly a single layer having a small layer thickness, and it has been difficult to increase the breakdown voltage, although the wire has an excellent flexibility.
U.S. Pat. No. 2,105,166 (Schwarzkopf), Jan. 11, 1938, discloses an electrical heating element with an electrical heating wire of molybdenum, tungsten, or tantalum forming a refractory metal core surrounded by a sintered cover containing an inner portion of metal oxide from groups 2, 3, and 4, except silicon, of the periodic table of elements, in contact with the core and an outer portion in which the metal oxide forms the major proportion and an addition of oxygen containing silicon compound. The inner metal oxide of the cover shall have a melting point higher than 1600.degree. C. and the silicon compound of the outer portion shall have a melting point sufficiently lower than 1600.degree. C. so that all the oxide present is sintered into gas-tight fragments. The sintering takes place at temperatures within the range of about 1400.degree. C. to 2200.degree. C. As a result, the conductor core must have a melting point sufficiently high to withstand these sintering temperatures. Copper and similar electrical conductor metals can thus not be used in the teaching of Schwarzkopf.
U.S. Pat. No. 2,975,078 (Rayfield), Mar. 14, 1961, discloses a method for producing an electrical conductor wire coated with a ceramic for use in a high temperature environment. The core is copper which is first provided with a nickel coating that is heated to form a nickel oxide on the surface of the copper core. Thereafter, a ceramic powder in the form of a so-called "slip" is applied to the nickel oxide surface and the ceramic powder on the wire is heated to produce a fused ceramic coating that is bonded to the copper wire core through the nickel oxide coating. The fusing or sintering takes place at a temperature in the range of 1600.degree. F. to 1800.degree. F. The ceramic coating of Rayfield is of the "vitrious enamel type of ceramic" produced of metallic oxides that form a glass type coating by fusion. The ceramic "slip" for producing the fusion glass comprises among other metal oxides a substantial proportion of lead oxide, namely 20% to 45% to keep the resulting fused glass coating pliant. However, such fused lead glass coatings have a relatively low melting point and the electrical insulation of this fused lead glass decreases significantly at a temperature exceeding 800.degree. C. Besides, such a fused lead glass is relatively coarse grained and hence porous. These features of Rayfield leaves room for improvement.